I believe Zmodeler (http://www.zmodeler2.com/) is used for the 3D stuff
Adobe Photoshop, paint.net, GIMP or equivalent for skins
notepad or other text editing software for.... sii config files
and some kind of compression software winzip; or the like (that can do "store") for the scs files
Older games you can use compression software for the the decompression of base.scs but as of ETS2 you need SCS's free tool to do so.

The maps are edited from within the game itself once you get the console active you would just type in "editor".
I myself don't know the details of much of the above but it might be a start for you.